Photometric variability in the old open cluster M 67

Free access article

Issue		A&A Volume 382, Number 3, February II 2002


Page(s)		899 - 909
Section		Stellar clusters and associations
DOI		http://dx.doi.org/10.1051/0004-6361:20011737

A&A 382, 899-909 (2002)
DOI: 10.1051/0004-6361:20011737

II. General survey

K. G. Stassun¹, M. van den Berg², R. D. Mathieu¹ and F. Verbunt³

¹ Department of Astronomy, University of Wisconsin, 475 N Charter St, Madison WI 53706, USA
² Osservatorio Astronomico di Brera, Via E. Bianchi 46, 23807 Merate (LC), Italy
³ Astronomical Institute, PO Box 80000, 3508 TA Utrecht, The Netherlands

(Received 26 September 2001 / Accepted 5 December 2001 )

Abstract
We use differential CCD photometry to search for variability in BVI among 990 stars projected in and around the old open cluster M 67. In a previous paper we reported results for 22 cluster members that are optical counterparts to X-ray sources; this study focuses on the other stars in our observations. A variety of sampling rates were employed, allowing variability on time scales ranging from ~0.3 hours to ~20 days to be studied. Among the brightest sources studied, detection of variability as small as $\sigma\approx 10$ mmag is achieved (with > $3\sigma$ confidence); for the typical star observed, sensitivity to variability at levels $\sigma\approx 20$ mmag is achieved. The study is unbiased for stars with 12.5<B<18.5, 12.5<V<18.5, and 12<I<18 within a radius of about 10 arcmin from the cluster centre. In addition, stars with 10<BVI<12.5 were monitored in a few small regions in the cluster. We present photometry for all 990 sources studied, and report the variability characteristics of those stars found to be variable at a statistically significant level. Among the variables, we highlight several sources that merit future study, including stars located on the cluster binary sequence, stars on the giant branch, blue stragglers, and a newly discovered W UMa system.

Key words: stars: activity -- stars: variables: general -- open clusters and associations: individual: M 67

Offprint request: K. Stassun, keivan@astro.wisc.edu

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